Caviomorphs stand out from other small rodents by producing extremely precocial young. Pups begin to nibble solid food right after birth and can survive weaning when 5 d old. We tested whether these properties of young lead to a low cost of lactation in guinea pigs (Cavia porcellus). In contrast to altricial rodents, energy acquisition and expenditure of guinea pig mothers (litter size, three) was highest during the first half of the 23-d lactation period. Daily metabolizable energy intake at peak lactation was increased by 126% above nonreproducing level and was 65% of the predicted maximal intake. peak daily energy expenditure was 1.9 times basal metabolic rate, and resting energy expenditure was elevated 26% above nonreproductive level. From the third day, young consumed solid food and intake increased linearly over the course of lactation. During lactation, total intake of litters through solid food consumption (assuming efficient digestion) amounted to 86% of the energy intake that mothers allocated to reproduction during that period. The pattern of energy expenditure during lactation in guinea pigs differs substantially from that of altricial rodents and is made possible because pups contribute through solid food intake. Overall energy demand on the mothers is not much lower than in altricial rodents, but lower peak energy demand during lactation in guinea pigs may reduce the coupling of reproduction to periods of high food abundance.